Mercury: photo mosaicNASA/JPLThe difficulty in seeing it notwithstanding, Mercury was known at least by Sumerian times, some 5,000 years ago. In Classical Greece it was called Apollo when it appeared as a morning star just before sunrise and Hermes, the Greek equivalent of the Roman god Mercury, when it appeared as an evening star just after sunset. Hermes was the swift messenger of the gods, and the planet’s name is thus likely a reference to its rapid motions relative to other objects in the sky. Even in more recent eras, many sky observers passed their entire lifetimes without ever seeing Mercury. It is reputed that Nicolaus Copernicus, whose heliocentric model of the heavens in the 16th century explained why Mercury and Venus always appear in close proximity to the Sun, expressed a deathbed regret that he had never set eyes on the planet Mercury himself.

Messenger: MercuryNASA/JHU/APL/Carnegie Institution of WashingtonUntil the last part of the 20th century, Mercury was one of the least-understood planets, and even now the shortage of information about it leaves many basic questions unsettled. Indeed, the length of its day was not determined until the 1960s, and, even after the flybys of the Mariner 10 and Messenger (Mercury Surface, Space Environment, Geochemistry, and Ranging) probes, the surface of the planet had not been completely observed. At first glance the portion of the planet that has been imaged looks similar to the cratered terrain of the Moon, an impression reinforced by the roughly comparable size of the two bodies. Mercury is far denser, however, having a metallic core that takes up about 61 percent of its volume (compared with 4 percent for the Moon and 16 percent for Earth). Moreover, its surface shows significant differences from lunar terrain, including a lack of the massive dark-coloured lava flows known as maria on the Moon and the presence of buckles and scarps that suggest Mercury actually shrank during some period in its history. Mercury’s nearness to the Sun gave scientists bound to Earth many observational hurdles, which were only overcome by the Messengerspacecraft mission. Messenger was launched in 2004, flew past the planet twice in 2008 and once in 2009, and settled into orbit in 2011. It mapped the entire surface of Mercury before crashing into the planet in 2015. Mercury’s proximity to the Sun has also been exploited to confirm predictions made by relativity theory about the way gravity affects space and time.

Planetary data for Mercury

mean distance from Sun

57,909,227 km (0.39 AU)

eccentricity of orbit

0.2056

inclination of orbit to ecliptic

7.0°

Mercurian year (sidereal period of revolution)

87.97 Earth days

maximum visual magnitude

−1.9

mean synodic period*

116 Earth days

mean orbital velocity

47.36 km/sec

radius (mean)

2,439.7 km

surface area

74,797,000 km2

mass

3.30 × 1023 kg

mean density

5.43 g/cm3

mean surface gravity

370 cm/sec2

escape velocity

4.25 km/sec

rotation period (Mercurian sidereal day)

58.646 Earth days

Mercurian mean solar day

175.9 Earth days

inclination of equator to orbit

0°

magnetic field strength

0.003 gauss

mean surface temperature

440 K (332 °F, 167 °C)

surface temperature extremes

700 K (800 °F, 430 °C); 90 K (−300 °F, −180 °C)

typical surface pressure

about 10−15 bar

number of known moons

none

*Time required for the planet to return to the same position in the sky relative to the Sun as seen from Earth.

Basic astronomical data

Mercury is an extreme planet in several respects. Because of its nearness to the Sun—its average orbital distance is 58 million km (36 million miles)—it has the shortest year (a revolution period of 88 days) and receives the most intense solar radiation of all the planets. With a radius of about 2,440 km (1,516 miles), Mercury is the smallest major planet, smaller even than Jupiter’s largest moon, Ganymede, or Saturn’s largest moon, Titan. In addition, Mercury is unusually dense. Although its meandensity is roughly that of Earth’s, it has less mass and so is less compressed by its own gravity; when corrected for self-compression, Mercury’s density is the highest of any planet. Nearly two-thirds of Mercury’s mass is contained in its largely iron core, which extends from the planet’s centre to a radius of about 2,100 km (1,300 miles), or about 85 percent of the way to its surface. The planet’s rocky outer shell—its surface crust and underlying mantle—is only some 300 km (200 miles) thick. For additional orbital and physical data, see the table.

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